WO2012031917A1 - Spectromètre thz et procédé de spectrométrie thz - Google Patents

Spectromètre thz et procédé de spectrométrie thz Download PDF

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Publication number
WO2012031917A1
WO2012031917A1 PCT/EP2011/064758 EP2011064758W WO2012031917A1 WO 2012031917 A1 WO2012031917 A1 WO 2012031917A1 EP 2011064758 W EP2011064758 W EP 2011064758W WO 2012031917 A1 WO2012031917 A1 WO 2012031917A1
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Prior art keywords
frequency
thz
optical
radiation
frequencies
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PCT/EP2011/064758
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German (de)
English (en)
Inventor
Markus GRÜNINGER
Joachim Hemberger
Axel Roggenbuck
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Universität Zu Köln
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Publication of WO2012031917A1 publication Critical patent/WO2012031917A1/fr

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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
    • G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
    • G01J3/28—Investigating the spectrum
    • G01J3/42—Absorption spectrometry; Double beam spectrometry; Flicker spectrometry; Reflection spectrometry
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
    • G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
    • G01J3/02—Details
    • G01J3/10—Arrangements of light sources specially adapted for spectrometry or colorimetry
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
    • G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
    • G01J3/28—Investigating the spectrum
    • G01J3/42—Absorption spectrometry; Double beam spectrometry; Flicker spectrometry; Reflection spectrometry
    • G01J3/433—Modulation spectrometry; Derivative spectrometry
    • G01J3/4338—Frequency modulated spectrometry
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
    • G01N21/25—Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands
    • G01N21/31—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry
    • G01N21/35—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light
    • G01N21/3581—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light using far infrared light; using Terahertz radiation
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
    • G01N21/25—Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands
    • G01N21/31—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry
    • G01N21/35—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light
    • G01N21/3504—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light for analysing gases, e.g. multi-gas analysis

Definitions

  • the invention relates to a THz spectrometer according to the preamble of claim 1 and 28.
  • the invention further relates to a THz spectroscopy method according to the preamble of claim 22 and 29.
  • THz spectroscopy has developed in recent years from purely scientific approaches to application-oriented orientations. Applications of THz spectroscopy are currently being discussed with regard to imaging techniques, in particular for detecting explosives or toxic gases. In addition, applications in astrophysics, molecular physics and solid state physics are discussed. A rapid development is in the generation and detection of THz radiation by means of optical coherent sources, e.g. Laser, determine. Two fundamentally different approaches to THz spectroscopy can be recognized, namely, in the time domain with conventional time-pulsed THz sources and in the frequency domain with CW-THz sources.
  • Temporally resolved spectroscopy naturally has narrow limits if comparatively high spectral resolution is to be achieved, and is based on the use of still comparatively expensive time-pulsed systems with a broad range Frequency spectrum instructed.
  • CW-THz spectroscopy readily enables the realization of a comparatively high spectral resolution (for example in the range of one MHz) with a simultaneously large bandwidth (for example in the range of a few THz) with comparatively inexpensive devices and still acceptable measurement costs, but this too is to be improved.
  • an approach has proved to be particularly suitable which can detect the amplitude and phase of a THz radiation in the context of CW-THz spectroscopy.
  • the central wavelengths of the DFB laser diodes at, for example, 853 nm and 855 nm allow the generation of two mutually detuned optical frequencies ⁇ 1 and ⁇ 2 and thus a mixing or difference frequency ⁇ 1 - ⁇ 2 from 0 to about 2 THz. It can be generated with a suitable photomischenden transmitter and photomischenden receiver CW-THz radiation of the difference frequency in the range between 60 GHz and 1, 8 THz and transmitted and detected via a CW-THz free-jet path.
  • the receiver is integrated to interferometrically superimpose a CW-THz radiation of the mixing frequency ⁇ 1 - ⁇ 2 with a beat of the optical radiation, thus homodyne and THz-phase-dependent photocurrent to eat.
  • An amplitude and phase of the CW-THz radiation can be determined from the photomust measured in this way.
  • the said THz phase difference is varied between the mixing frequency of the optical radiation in a branch of an interferometer and a received CW-THz radiation by scanning the mixing frequency and thus also the CW-THz frequency at different optical lengths of the first and second branches -
  • the photo-Ström varies with the CW-THz frequency, from which the THz phase can be determined.
  • the change in the CW THz frequency in small frequency steps is required, resulting in a comparatively long measurement time. Since both the amplitude and the phase of the CW-THz radiation is determined without having to vary a CW-THz radiation regime, this approach has proven particularly useful.
  • a determination of the amplitude and phase of the CW-THz radiation is in particular a prerequisite for determining a complex dielectric function, eg. As a solid by means of CW-THz spectroscopy, but this is not possible in a number of other known methods.
  • an amplitude and phase-sensitive THz spectroscopy is also known from US Pat. No. 6,348,683 B1 or to Deninger et al. in Rev. Sei. Instr. 79, 044702 (2008). Deninger et al.
  • the amplitude and phase of the CW-THz radiation should be determinable by varying a phase difference to an optical mixing frequency. It is desirable to improve a CW-THz spectrometer for determining an amplitude and phase of the CW-THz radiation, in particular by measuring a photocurrent, in terms of practicability, in particular measuring speed, precision and stability. An achievable frequency broadband with simultaneously high frequency resolution should not be affected as far as possible.
  • the following generally refers to a THz spectrometer and a THz spectroscopy method, regardless of their particular suitability for CW-THz application.
  • the described and claimed THz spectrometer and a THz spectroscopy method for a CW-THz application is suitable.
  • a use of the described The THz spectrometer and the THz spectroscopy method for time-resolved THz spectroscopy are not excluded.
  • the object of the invention is to specify a THz spectrometer and a THz spectroscopy method with which it is possible to determine the amplitude and phase of a THz radiation and which is improved in terms of practicability, in particular measuring speed, precision and stability. In particular, a measurement effort should be reduced comparatively.
  • the THz spectroscopy is advantageously possible in a comparatively broadband frequency range with comparatively high spectral resolution.
  • the object with respect to the THz spectrometer is achieved by a THz spectrometer, in particular CW THz spectrometer, with the features of claim 1 and 28.
  • the object with respect to the method is achieved by a THz spectroscopy method, in particular CW-THz spectroscopy , with the features of claim 22 and 29.
  • the invention is based on the consideration that for the complete determination of the complex dielectric function, z. B. a solid to be examined, the amplitude and phase of a THz radiation, in particular CW-THz radiation should be determinable.
  • the concept of the invention is defined independently of the generating means for the THz radiation. Basically, there are a variety of ways - such. B. by per se known BWO oscillators (back ward wave oscillator) - to generate coherent THz radiation directly at a THz frequency. In particular, opto-electronic generators have also proven themselves. In a second variant of claims 1 and 22 is provided to generate the THz radiation by means of opto-electronic means.
  • the THz spectrometer accordingly has, in a first branch, an opto-electronic mixer in the form of a transmitter, with which THz radiation can be generated from the mixing frequencies of the optical frequencies of the first branch.
  • the second branch has an opto-electronic mixer in the form of a receiver with which the THz radiation can be received.
  • the THz radiation is in particular superimposed with the mixing frequencies of the optical frequencies of the second branch receivable.
  • the THz radiation can be transmitted directly from the transmitter to the receiver without beam guiding means in the free jet.
  • a THz radiation guide can be provided with beam guiding means, such as mirrors, in particular parabolic mirrors, or the like, between transmitter and receiver.
  • the THz spectrometer has a modulator arrangement with at least one modulator, by means of which a phase difference between a mixing frequency of the optical frequencies of the second branch and the THz radiation is variable.
  • the beat signal on the mixing frequency is designated in each case.
  • a phase difference between a beat on the mixing frequency of the optical frequencies of the second branch and the THz radiation is varied.
  • a detector is provided by means of which a receiver signal can be detected as a function of the varied phase difference for determining at least one phase, in particular the phase and / or amplitude of the THz radiation.
  • Particularly advantageous phase and amplitude is detected.
  • only the phase or only the amplitude can be detectable.
  • the detector is formed with a photocurrent detector by means of which a photocurrent is detectable at the receiver to form the receiver signal.
  • the source arrangement has a, in particular coherent, optical source of the first and second type.
  • A, in particular coherent, optical source of the first type is designed to generate optical radiation of at least a first and a second discrete frequency.
  • the optical source can have one or two units, in particular a coherent optical unit such as a laser or the like.
  • the invention has recognized that the use of discrete frequencies has significant advantages, e.g. B. in terms of computational effort and speed in the evaluation of the receiver signal.
  • The, in particular coherent, optical source of the second type is designed to generate optical radiation of at least one third optical frequency.
  • This may be a single third discrete optical frequency fixed or tunable relative to the first and second optical frequencies.
  • There may also be a plurality of third optical discrete frequencies, e.g. B. in the context of a frequency comb of discrete frequencies may be provided.
  • a frequency comb with suitable use can practically replace a measurement with a continuous spectrum and nevertheless a sufficient frequency resolution is possible.
  • the concept of the invention is based on the use of discrete frequencies and thereby achieves significant practicality advantages, without having to sacrifice frequency resolution.
  • a discrete frequency is to be understood as meaning a sufficiently narrow-band line frequency with an intensity maximum at a center frequency which is due to its intensity maximum and line width is clearly distinguishable from other discrete frequencies.
  • a discrete frequency does not form an indistinguishable mere frequency component in a continuous broadband white-light frequency spectrum, but can be obtained from such as eg by a filter or the like.
  • a line width of a discrete frequency is so low that an interferometric superposition with other discrete frequencies can be achieved in the spectrometer, that is, a phase difference can be varied and detected by means of the modulator arrangement.
  • the, in particular coherent, optical sources of the first and second type differ in particular in that the optical source of the first type (I) is designed to generate the optical radiation of at least the first and second optical discrete frequencies ( ⁇ 1, ⁇ 2) is.
  • a frequency stabilization means is provided such that at least one frequency-stabilized mixing frequency ( ⁇ 1- ⁇ 2) can be formed.
  • the frequency stabilizer may, but need not, be provided in the first type optical source (I).
  • the first and second optical frequencies are absolutely frequency-stabilized. But it can also be a distance of the same relative - ie the mixing frequency ( ⁇ 1- ⁇ 2) -frequency-stabilized.
  • a frequency-stabilized mixing frequency is to be understood as meaning a mixing frequency which, in any case, is not actively changed. Accordingly, a frequency stabilization means serves to keep a frequency-stabilized mixing frequency largely constant over a time scale appropriate to the measurement period. In particular, a frequency-stabilized mixing frequency is constant, or is kept constant while the tunable mixing frequencies are changed, in contrast to the said further possibly. Tunable mixing frequencies.
  • the invention has recognized that the acquisition of spectra with determination of the amplitude and phase of a THz radiation requires a certain amount of time, which is considerably reduced with the present concept. Nevertheless, within the reduced time overhead, the spectrometer may undergo drift that is correctable with the concept of the invention.
  • the concept according to the invention assumes that drifts occurring in the phase or amplitude of a THz radiation for the frequency-stabilized mixing frequency are largely attributable to a drift of the spectrometer. Live and Amplitudendrifts at the frequency-stabilized mixing frequency can therefore be used according to the concept of the invention as a correction measure for other mixing frequencies.
  • a phase cp 0 and an amplitude A 0 at a time T 0 of a measurement start or as an average value for a THz radiation, in particular CW-THz radiation, at the stabilized measurement frequency ( ⁇ 1- ⁇ 2) can be defined particularly advantageously.
  • the evolution of the phase cp, and amplitude A, at a later time T, relative to the phase cp 0 and the amplitude A 0 for the THz radiation, in particular CW-THz radiation, at the stabilized mixing frequency ( ⁇ 1- ⁇ 2) thus a fictitious drift-related development of a phase cp 0 to cp, and A 0 to A, at.
  • this drift-related development is to be taken as a correction measure in a comparison of the phase and amplitude of the THz radiation, in particular CW-THz radiation, of the further discrete mixed frequencies.
  • this evolution of phase and amplitude of a CW-THz radiation at the stabilized mixing frequency is subtracted from a measured phase and amplitude of the CW-THz radiation at the other discrete mixing frequencies to produce a truly reliable value of amplitude and amplitude Phase to come at the further discrete mixing frequency.
  • The, in particular coherent, optical source of the second type (II) is designed to generate optical radiation of at least one third optical discrete frequency ( ⁇ 3, ⁇ 3,) and may, but need not, be frequency-stabilized.
  • an optical unit thereof may be tunable over a wider spectral range or may also generate a plurality of discrete frequencies.
  • the concept sees the simultaneous guidance of THz radiation, in particular CW-THz radiation, from the mixing frequencies ( ⁇ 1- ⁇ 2, ⁇ 1- ⁇ 3 or ⁇ 3, ⁇ 2- ⁇ 3 or ⁇ 3,) of the first and second optical frequencies ( ⁇ 1, ⁇ 2) and the third optical frequency ( ⁇ 3, ⁇ 3,).
  • An evaluation unit connected to the detector is designed to measure the phase and amplitude of the THz radiation, in particular CW-THz radiation, at the stabilized mixing frequency ( ⁇ 1- ⁇ 2) and at least one, in particular two, further mixing frequencies ( ⁇ 1- ⁇ 3 or ⁇ 3 ,, ⁇ 2- ⁇ 3 or ⁇ 3,) and to compare with each other.
  • the stabilized mixing frequency ( ⁇ 1- ⁇ 2) - as initially generally constant and discrete reference frequency, in particular over a measuring period stable discrete reference frequency - together with other mixing frequencies ( ⁇ 1- ⁇ 3 or ⁇ 3 ,, ⁇ 2 - ⁇ 3 or ⁇ 3,) - as possibly tunable test frequency (s) - be performed simultaneously.
  • Amplitude and phase at the stabilized reference frequency can thus be measured during an entire measurement time and are available for each measurement time as a correction measure.
  • a Measurement with the reference frequency is thus available as an exact (since simultaneously obtained) correction measure for a measurement with the test frequency.
  • a correction measure is also provided for a fast drift of the spectrometer, which until now could only be eliminated by time-consuming averaging.
  • This also applies in particular to a correction measure for a thermal drift or the like.
  • Slow drift of the spectrometer which occurs during a measuring period.
  • the reference frequency is provided with an advantage particularly not susceptible to drift of the spectrometer.
  • the spectrometer, in particular the source arrangement can for this purpose have frequency stabilizing means, which in any case provide the stabilized mixing frequency ( ⁇ 1- ⁇ 2) -as a constant, in particular stable, and discrete reference frequency-independent of a fast or slow drift. A drift or other inaccuracies can thus be compensated by the simultaneously obtained measurement with the reference frequency.
  • a reference measurement as a result of the three optical frequencies used with improved corrective measure is already possible at the same time as a sample measurement.
  • An evaluation unit connected to the detector is suitably designed to at least the phase, in particular phase and amplitude of the THz radiation, in particular CW-THz radiation, at at least one, in particular two, further mixing frequencies ( ⁇ 1- ⁇ 3 or ⁇ 3 ,, ⁇ 2- ⁇ 3 od. ⁇ 3,) and the stabilized mixing frequency ( ⁇ 1- ⁇ 2) to be determined and compared with each other according to the invention; especially in general terms.
  • a comparison is in particular to be understood as any measure which comprises at least the phase, in particular the phase and amplitude, of the THz radiation at at least one, in particular two, further mixing frequencies ( ⁇ 1- ⁇ 3 or ⁇ 3 ,, ⁇ 2- ⁇ 3 or ⁇ 3, ) with those at the stabilized mixing frequency ( ⁇ 1- ⁇ 2) corrected. This is possible due to the discrete frequencies used with relatively little computational effort. In a modification, only the phase or only the amplitude can be evaluated.
  • three, in particular coherent, optical units can be used to implement the concept of the invention with the formation of three discrete mixing frequencies, wherein at least one of the mixing frequencies serves as a reference frequency for performing a simultaneous correction or reference measurement, which in Reference is made to a sample measurement with at least one of the other mixing frequencies as sample frequencies.
  • the cover particularly compensates for a drift of the Spectrometer during the measurement.
  • the use of discrete mixing frequencies can be used to a considerable reduction of the measuring time.
  • coherent optical radiation is used to generate phase information carrying THz radiation.
  • optical radiation is meant in particular radiation in the UV, VIS, NIR, IR frequency range.
  • optical radiation in the NIR range in particular in the range between 750 nm and 900 nm and 1500-1900 nm due to the availability of suitable laser diodes and optoelectronic mixers with high electron mobility, z. B. based on LT-GaAs or ion-implanted GaAs proved to be advantageous.
  • For the application are also particularly interesting optical sources at wavelengths in the range of 1550 nm for the generation of THz radiation, z.
  • opto-electronic mixing components based on InGaAs or the like.
  • the spectrometer proves to be comparatively broadband usable in the range of a CW THz frequency between about 60GHz and about 2Thz.
  • a spectral resolution of the same is limited only by the line width of the optical sources used first and second type and goes to a range well below 100 MHz. As such, the spectrometer can be used compactly and with advantageously reduced measuring time.
  • the evaluation unit has a module for determining a drift of at least one of the phases, in particular the phase and amplitude ( ⁇ 0 , A 0 , ⁇ ,, A,) of the THz radiation at the stabilized mixing frequency ( ⁇ 1 - ⁇ 2).
  • the determination of a drift can also relate only to the phase or only to the amplitude.
  • the module is in particular designed to determine a temporal drift profile of at least the phase, in particular phase and / or amplitude ( ⁇ ,, A,).
  • the change of at least the phase, in particular phase and / or amplitude, of the THz radiation at the stabilized mixing frequency can advantageously be determined.
  • phase and amplitude profile obtained in this way can advantageously be available as a correction measure.
  • a mean value of the phase and amplitude ( ⁇ 0 , A 0 ) from the drift profile can be determined comparatively easily and be available as a correction measure.
  • the THz spectrometer has an adjusting unit which is designed to detune the at least one further mixing frequency ( ⁇ 1- ⁇ j3od.uj3i, cjj2- ⁇ j3d.cjj3i) in the region of the THz spectrum to accommodate a THz spectrum and thereby stabilize the one Mixing frequency ( ⁇ 1- ⁇ 2) to keep constant.
  • So mixed frequencies (cjj1-üj3od.cjj3i, cjj2-üj3od.cjj3i) for receiving a THz spectrum for example, be selected in a range between 0.1 to 2THz, as explained in a particularly preferred embodiment.
  • a correction measure for at least the measured phase, in particular phase and / or amplitude, of the THz radiation also at the other mixing frequencies is advantageously available according to the development.
  • the invention provides in the source arrangement a coherent optical source of the second kind, separate from the first type coherent optical source.
  • the latter is designed in comparison to the former for generating optical radiation of at least a third optical frequency.
  • the third frequency is tunable relative to the first and second optical frequencies in the context of this development.
  • the tunable third frequency may, in the context of the concept, be in particular a centroid frequency of a narrowband line frequency.
  • the comparatively narrowband line frequency can be tuned over a comparatively broadband frequency range.
  • the third frequency may advantageously be part of a frequency comb of, in particular, equidistant voice frequencies thereof with given line spacing.
  • the coherent optical source of the first type is advantageously comparatively stable for generating optical radiation of at least one of a first and a second optical frequency, in order to provide a reference frequency by means of which a correction measure for amplitude and phase are obtained even at other mixing frequencies as sample frequencies can.
  • the coherent optical source of the second type offers the possibility of generating test frequencies in a comparatively broad spectrum and / or tunable.
  • a modulator is advantageously provided which generates the phase difference by means of an optical path length difference.
  • a modulator may, for example, be realized as a mechanical delay unit or advantageously as a fiber stretcher.
  • An amplitude of an optical path length difference can be advantageously made particularly large due to the relatively large stroke of a fiber stretcher.
  • a fiber stretcher can be implemented comparatively compact and with little adjustment effort in a fiber-bonded optical radiation guide. Compared to a - theoretically also possible - mechanical delay unit can also be reduced with a fiber stretcher a measurement time.
  • a fiber stretcher offers the possibility comparatively easy - especially in the aforementioned spectrally broadband or tunable design of the spectrometer using a frequency comb for the third optical frequency - to realize a relatively small frequency spacing in the frequency comb.
  • a stroke of a fiber stretcher is particularly advantageous for resolving discrete frequencies of a frequency comb, since, in principle, an achievable frequency resolution increases with the amplitude of an optical path length difference.
  • the amplitude and rate for varying a phase difference can be increased.
  • the concept of the development provides in combination for an improvement of the amplitude and phase-sensitive detection of THz radiation in terms of the spectral broadband and spectral resolution.
  • a particularly high precision and stability of the THz spectrometer is achieved.
  • a fiber stretcher z. B. provide symmetrically in both branches of the spectrometer, which compensates by the fiber stretcher caused by drift effects compensating per se.
  • a laser diode or a similar optical unit is suitable for forming a coherent optical source of the first and / or second type, since this can be implemented in a comparatively compact and cost-efficient manner in a spectrometer.
  • a laser diode can be coupled particularly advantageous to a fiber guide.
  • an optical part of the spectrometer entirely based on fiber optics and laser diodes can be produced integrally and compactly.
  • An opto-electronic mixer can be relatively easily coupled to a fiber guide.
  • a particularly preferred unit for use with the first and / or second coherent optical source is at least one laser diode for generating optical radiation at at least one narrowband line frequency.
  • a DFB (distributed feedback) laser diode or ECDL (external cavity laser diode) laser diode e.g.
  • a DBR (distributed Bragg reflector) laser diode has been found to be advantageous for generating a particularly narrow band optical frequency.
  • Such or other laser diode is regularly with a wavelength selective feedback - z. B. by introducing a filter or the like, which results in a comparatively small spectral bandwidth of the emitted optical radiation.
  • this is a preferable alternative to other more elaborate wavelength selection methods for a laser diode, which is conducive to a compact design of the THz spectrometer.
  • the coherent optical source of the first and / or second type comprises a frequency-tunable laser diode as an optical unit.
  • a laser diode in particular a DFB or DBR laser diode, be provided with a frequency tuning means.
  • a frequency sweeping means in the form of a current and / or temperature adjusting means is suitable.
  • about the temperature of a diode, with accuracy z. B. in the mK range, can be a laser-active length of the diode over a relatively large range set.
  • about the adjustable with the current number of charge carriers can be a refractive index of the diode set. Overall, thus an emission frequency of the laser diode is adjustable, z. B.
  • DFB laser diode in the range between 750 nm and 900 nm or 1500 nm and 1700 nm.
  • a tunable in comparatively narrow ranges DFB laser diode for example, in the range between 853 and 855 nm tunable.
  • Another range may be selected as needed by selecting the laser active diode material.
  • the tunability of a laser diode used is not limited to a certain frequency range but can be selected as needed.
  • the coherent optical source of the first kind in particular a laser diode or a periphery thereof, with a frequency stabilizer.
  • the concept of the invention offers the possibility of directly stabilizing a mixing frequency using a suitable stabilizing agent. So z. B. using the above tunability of laser diodes, the frequencies ( ⁇ 1, ⁇ 2) of the optical source of the first type (I) with a fixed distance of a mixing frequency ( ⁇ 1 - ⁇ 2) are detuned to a drift of the mixing frequency ( ⁇ 1 - ⁇ 2) prevent.
  • a frequency-stabilized mixing frequency is achieved in that already a first and second optical frequency is controlled as stable as possible in each case with respect to a desired frequency; inasmuch as an absolute value of the first and second optical frequencies is stabilized.
  • a frequency stabilization means can be realized, for example, in the form of an active or passive stabilization of the laser diode. This advantageously leads to a thermal drift of the first and second optical frequencies, but in any case of the mixing frequency, being kept as low as possible in the case of temporally spaced measurements.
  • a drift for at least the first THz frequency ⁇ 1 - ⁇ 2 - as a mixing / difference frequency between see the first and second optical frequency - is minimized.
  • a drift of the spectrometer is basically due to z.
  • the source arrangement provides for two separate coherent optical units of the first type source, for example in the form of two DFB laser diodes.
  • a first of the DFB laser diodes can be adjusted to a first optical frequency comparatively narrow band and stable.
  • a second of the DFB laser diodes can be adjusted to a second optical frequency comparatively narrow-band and stable.
  • the coherent source of the first kind can also by means of a single laser -.
  • the at least one modulator is arranged in the fiber guide such that the phase difference can be generated by means of an optical path length difference of the first branch to the second branch.
  • the fiber guide is designed to guide optical radiation having at least the first, second and third optical frequencies.
  • the transmitter as well as the receiver and the THz radiation guide are preferably designed for transmitting a THz radiation with a first, second and third mixing frequency of a THz radiation formed as a function of the third frequency.
  • the fiber guide has a coupling point in which the third optical frequency can be coupled in.
  • a coupling point can advantageously be realized as a separate fiber coupler between a fiber coupler for the first and second optical frequency and the opto-electronic mixers.
  • the fiber guide has only a single coupling point for coupling in the third optical frequency.
  • the spectrometer is particularly preferably designed as an interferometer with which a homodyne, coherent detection of THz radiation is possible.
  • such an interferometer can basically be constructed on the principle of a Fourier transform interferometer.
  • a photocurrent detector is preferably coupled to an evaluation unit which has a module for Fourier transformation of the photocurrent measured as a function of the phase difference.
  • the evaluation unit can have a module by means of which a trigonometric interpolation and / or regression of a photocurrent signal is possible.
  • a corresponding oscillation signal of a first, second and third THz frequency can be comparatively easily interpolated with respect to the three optical discrete frequency values of the mixing frequencies or in the frame adjust a regression with least squares.
  • the free parameters of a phase and amplitude obtained at such an adaptation at each individual frequency of the THz radiation are particularly easily determinable in this way - a costly Fourier transformation can advantageously be dispensed with.
  • the coherent optical source of the second type has an optical unit as a tunable single-mode source, which is designed for a comparatively wide optical frequency range, ie for generating a single voice frequency as tunable third optical Frequency.
  • a comparatively narrow-band DFB laser diode can be used for this purpose, which supplies a single third optical frequency, which can be tuned or displaced in a comparatively large spectral range.
  • the second type coherent optical source has an optical unit as a multi-mode source for a comparatively wide frequency range, ie in particular for generating a frequency comb with a plurality of voice frequencies, is formed.
  • the use of three optical frequencies according to the concept of the invention with a first and second high-precision and stable optical discrete frequency and a broadband tunable third optical discrete frequency leads to the fact that three THz frequencies for the THz spectra meter are available - namely to form a first stable and precise reference mixing frequency and two tunable further mixing frequencies as measurement frequencies.
  • the aforementioned further provides a plurality of measurement frequencies by using the plurality of voice frequencies of the frequency comb available. You can use the voice frequencies to redundantly determine a measurement spectrum or to reduce the measurement time for a full frequency spectrum.
  • a large number of voice frequencies of the frequency comb with a small frequency spacing can be used over the entire spectrum. It can be reduced in the context of a development, a frequency separation of the voice frequencies when a modulator, z. B. as a fiber stretcher, provided with a comparatively large stroke.
  • a modulator, z. B. as a fiber stretcher, provided with a comparatively large stroke.
  • a phase difference by means of an optical path length difference of the first to the second branch of the optical radiation guide for a resolution of 1 GHz an optical path length difference of about 30 cm is required.
  • a multimode source with a frequency comb a variety of z. B.
  • 25 GHz-spaced voice frequencies is already an optical path length difference of about 1 cm is sufficient to spectrally separate the voice frequencies associated with each measurement information.
  • an optical path length difference can be further reduced. The less sensitive the spectrometer is, the lower the optical path length difference can be.
  • the realization of a spectrometer with comparatively high spectral resolution is possible.
  • the spectral resolution initially depends on the characteristic of the optical source; the spectral width depends on the width of the frequency comb of the optical source of the second kind.
  • a spectrally comparatively high-resolution, broadband spectroscopy is made possible according to this development of a THz spectrometer.
  • a plurality of voice frequencies of the frequency comb with a line spacing in the range between 5 GHz to 30 GHz has been found, in particular in the range between 5 GHz to 10 GHz.
  • the tuning frequencies are preferably arranged equidistantly in the frequency comb.
  • An amplitude of an optical path length difference is particularly advantageously adjustable between 0.5 and 3 cm, in particular sufficiently, ie limited to this range.
  • the optical frequency comb is particularly preferably tunable, z. B. in the range of a few MHz to at least 100 MHz.
  • Particularly advantageous is a frequency comb over the line spacing of the tuning frequencies tunable. This advantageously avoids gaps in the spectrum.
  • the first and second optical frequencies can also be tuned if the mixing frequency is kept stable as a reference frequency.
  • a frequency resolution is advantageously in the MHz range or significantly lower.
  • a frequency resolution is independent of the stroke of a path length modulation, ie only dependent on the line width of the optical discrete frequencies used or the characteristics of the optical units used.
  • a modulator of the modulator arrangement can be operated in a so-called "step-scan mode" (ie alternately vary phase, then measure) .
  • This operating mode is suitable in particular for use in the context of a lock-in measurement
  • conventional mechanical path delay units are used to implement a modulator in a step-scan mode (ie, measurement data is acquired for a number of fixed delay steps)
  • the use of a continuous modulator provides that the path length difference is not limited to discrete ones Stages are changed, but moreover, it is continuously changed, thereby eliminating the idle time required for the step-scan measurement and thus reducing the total required measurement time.
  • the maximum amplitude of an optical path length difference can advantageously be doubled by arranging a modulator in each of the branches of the optical radiation guide.
  • a modulator arranged in a first branch and a modulator arranged in a second branch are preferably operated asymmetrically, so that an optical path length in a first branch is lengthened while the optical path length in a second branch of the optical radiation guide is reduced or vice versa.
  • a thermal drift of the optical radiation guide is compensated, i. H. an operation is possible in which the optical path lengths of the first and second branches drift in the same way, whereby the drifts compensate each other.
  • a modulator in the form of a fiber stretcher has proved to be particularly preferred since it allows a continuous change of the optical path length difference of the first to the second branch of the optical radiation guide.
  • a fiber stretcher allows asymmetric operation by applying to each other Inverse voltages to a first and second fiber stretcher in a first and second branch.
  • a symmetrical operation is also possible by applying voltages of the same sign to a first and second fiber stretcher in a first and second branch of the optical radiation guide.
  • a fiber stretcher can be operated with a comparatively high scanning rate.
  • a clock rate between 100 Hz and 1, 5 kHz, in particular between 300 Hz and 1, 2 kHz.
  • the fiber stretcher can even at relatively low scan rates, z. B. with clock rates optionally below 10 Hz, in particular below 1 Hz, operated.
  • the choice of a high clock rate and, associated therewith, the amplitude of a path length difference can preferably be selected as a function of a desired spectral resolution and in accordance with a line spacing in a frequency comb.
  • the choice of a low clock rate can be selected as a function of a desired spectral resolution and in accordance with a line spacing in a frequency comb.
  • a THz spectroscopy method it has proven to be particularly preferable that measurement data obtained from a photocurrent be used to determine a reference of phase and amplitude of the THz radiation, i. Measurement data with a sample to be examined are compared with reference data (without sample).
  • a reference Einstrahlspektrum can be generated.
  • a single-beam spectrum basically contains the spectral properties of an energy distribution of the light source, the transmission function of the spectrometer as well as the sensitivity of the detector.
  • a development of the THz spectroscopy method advantageously provides that measured data obtained from a photocurrent for determining the phase and amplitude of the THz radiation with the second or third THz frequency (as the difference frequency from the first ⁇ 1 and third ⁇ 3 or second ⁇ 2 and third ⁇ 3 optical frequency) are used to generate a measurement Einstrahlspektrums.
  • a measurement single-beam spectrum basically contains the spectral properties of an energy distribution of the light source, a transmission function of the spectrometer and the sensitivity of the detector and, if present, the spectral properties of a sample.
  • the measurement single-beam spectrum determined in this way can be made to be tunable, in particular for different measurement frequencies, if the third frequency ⁇ 3 is chosen to be tunable.
  • a measurement single-beam spectrum (obtained with second and / or third THz frequency) can basically be corrected for thermal drift via the reference data (obtained with the first THz frequency ⁇ 1 - ⁇ 2).
  • This measure proves to be particularly advantageous if the inclusion of a spectrum (consisting of many NEN, successively measured measurement frequencies) takes a longer period of time or if a longer period between the recording of a spectrum without a sample to be measured and the inclusion of a reference spectrum with sample is.
  • the problem associated with this is virtually eliminated according to the concept of the invention, since measurement data at ⁇ 1- ⁇ 3 and ⁇ 2- ⁇ 3 and reference data at ⁇ 1- ⁇ 2 are recorded simultaneously as a correction measure.
  • 1 shows a first preferred embodiment of a CW-THz spectrometer according to the concept of the invention with a simple modulator for modulating an optical path length difference (OPD) between a first and second branch of the optical radiation guide of the CW-THz spectrometer two variants with a single-mode optical source of the second kind (L3) or a multi-mode optical source of the second kind (L4) can be realized;
  • 2 shows a second embodiment of a CW-THz spectrometer according to the
  • a modulator arrangement for modulating an optical path length difference is formed by means of two fiber stretchers, wherein a first fiber stretcher is arranged in a first branch and a second fiber stretcher is arranged in a second branch of the optical radiation guide, and wherein the fiber stretcher is asymmetrical by means of inverse stresses
  • the embodiment can be implemented in two variants with a single mode optical source of the second type (L3) or a multi-mode optical source of the second type (L4);
  • FIG. 3 shows an example of a photo current detected at a first CW THz frequency at approximately 103 GHz - in view A, the photocurrent measured in a measuring setup of FIG. 2 using an optical path length difference (OPD) is shown, wherein the optical path Path length difference is generated with a fiber stretcher - in view B, the measured with a motorized path delay unit in the CW-THz radiation guide photocurrent is shown;
  • FIG. 4 shows in view A a voltage at a fiber stretcher of the construction of FIG.
  • the optical frequencies ⁇ 1, ⁇ 2, ⁇ 3 of the three near-infrared (NIR) lasers L1, L2, L3 in the present embodiments are in the range between 850 and 855 nm.
  • the concept of the invention exemplified in the drawing combines a high spectral resolution, which is practically limited only by the laser characteristics of the three lasers L1, L2, L3, as well as a comparatively large spectral bandwidth for a measurable CW-THz radiation, which reaches up to 2 THz - the latter is practically only by the spectral characteristics of the laser used L1, L2, L3 and the photomixer, ie THz transmitter and receiver limited. In all cases, a coherent homodyne detection of CW-THz radiation is possible with the interferometer described here.
  • the amplitude and phase of the CW-THz radiation at the frequencies ⁇ 1 - ⁇ 2, ⁇ 1 - ⁇ 3, and ⁇ 2 - ⁇ 3 become superposition with the mixing frequencies ⁇ 1 - ⁇ 2, ⁇ 1 - ⁇ 3, ⁇ 2 - ⁇ 3 from the three optical frequencies ⁇ 1, ⁇ 2, ⁇ 3 determined in the photomixing receiver 32.
  • the superimposition of the radiation of two lasers L1, L2, L3 creates a "beating" in the intensity.
  • the measurement accuracy is limited by in particular a thermal drift of the mechanical components of the spectrometer 100, 200, 300, 400, in particular the radiation guidance and the optical fiber.
  • This drift is important because typically a measurement of the sample to acquire a measurement spectrum and the measurement without a sample to acquire a reference spectrum are performed one after the other.
  • a time interval in between can with a drift of the CW-THz spectrometer -.
  • the radiation guide or the source arrangement - be connected, which thus leads to erroneous measurement data.
  • Simultaneous recording of a reference measurement and sample measurement is achieved in a particularly advantageous manner by the efficiently usable spectrometers 100, 200, 300, 400 explained with reference to FIGS. 1 and 2.
  • ⁇ 1 - ⁇ 2 is highly precise and largely stable, so that drift influences can be derived therefrom.
  • the lasers L1 and L2 are carried out with high precision and stability in the present case with a DFB laser diode which can be tuned between approximately 850 nm and 855 nm.
  • the third laser L3 delivers an optical frequency ⁇ 3, which is tunable comparatively broadband.
  • a particularly stable and precise mixing frequency is generated and two tunable CW THz frequencies ⁇ 1- ⁇ 3 and ⁇ 2- ⁇ 3, which serve as measuring frequencies and are also referred to as test frequencies.
  • the CW-THz spectrometer 100, 200 provides the aforementioned high-precision stable laser L1 and the high-precision stable laser L2 formed separately therefrom as separate lasers of a source arrangement of a coherent first type optical source I. Furthermore, the CW-THz spectrometer 100 sees the third laser L3 and the CW-THz spectrometer 200 the fourth laser L4 as broadband tunable laser to form a coherent second type II optical source.
  • the source arrangement 10 of the CW-THz spectrometer thus formed supplies at least three discrete optical frequencies ⁇ 1, ⁇ 2, ⁇ 3 and ⁇ 3, respectively, which are coupled into an optical radiation guide in the form of a fiber guide 20.
  • the fiber guide on a first optical coupling path 20.1 for coupling the first optical frequency ⁇ 1 and coupling the second optical frequency ⁇ 2 - then a second optical coupling path 20.2 for further coupling a third optical frequency ⁇ 3 in the path of the fiber guide 20.
  • a coupling path mentioned here can, for example, as a so-called fiber combiner or beam splitter or the like. be formed.
  • the fiber guide 20 provides a free-beam optic 27, which couples the fiber guide to the further optical beam guidance in the form of a free-jet guide 29.
  • the free-jet guide 29 has a first branch 21 and a second branch 22.
  • a single modulator is formed as an optical path free path optical delay unit 23 capable of detecting a phase difference between a mixing frequency ⁇ 1- ⁇ 2, ⁇ 1- ⁇ 3, ⁇ 2- ⁇ 3 of the optical radiation at ⁇ 1, ⁇ 2, ⁇ 3 and the CW-THz radiation 40 by means of an optical path length difference of the first branch 21 to the second branch 22 to produce.
  • An opto-electronic mixer in the form of a transmitter 31 for generating the CW-THz radiation 40 is coupled to the first branch 21, mixing the frequencies ⁇ 1, ⁇ 2, ⁇ 3 of the optical radiation.
  • an optoelectronic mixer in the form of a receiver 32 is coupled to the second branch 22, mixing the frequencies of the optical radiation in the second branch 22.
  • the opto-electronic mixers are formed by means of high-energy ion-implanted gallium arsenide with an active area of about 9 ⁇ 9 ⁇ m. Compared to LT gallium arsenide, this has the advantage that the photoconductive material is very reproducible.
  • the photomixers are integrated with a three-turn self-complementary log-periodic spiral broadband antenna. Such a design is sufficient to produce a nearly constant antenna impedance in the frequency range of about 100 GHz to well above 1 THz.
  • the opto-electronic mixers, ie transmitter 31 and receiver 32 are provided with a hyperhemispheric silicon lens 33.
  • the remaining beam divergence of the CW-THz radiation 40 at the transmitter and receiver 31, 32 is below 10 °.
  • the emitted CW-THz radiation 40 is focused via two extra-axial parabolic mirrors 41, 42, each with a focal length of 75 mm.
  • the sample P may be introduced into or removed from an aperture of the collimated CW-THz radiation 40.
  • the photocurrent in the receiver 32 is less than 1 ⁇ and in the range of an absorption band of the sample P or in the case of destructive interference in the homodyne coherent detection of the photocurrent near the noise level of about 4 pA. Due to the coherent detection, a signal-to-noise ratio (in terms of the THz intensity) of more than 90 dB at 100 GHz is achieved.
  • a CW-THz spectrometer 100, 200 shown in FIG. 1 provides for the acquisition of a reference measurement using the discrete frequencies ⁇ 1, ⁇ 2.
  • the phase difference between the CW-THz radiation 40 and the mixing frequency ⁇ 1- ⁇ 2 is modulated via an optical path length difference OPD between the first branch 21 and the second branch 22, which takes place by means of the optical path delay unit 23.
  • OPD optical path length difference
  • the phase and amplitude of the CW-THz radiation 40 can be determined very precisely and stably for the first mixing frequency ⁇ 1- ⁇ 2, ie CW-THz frequency thereof.
  • the precision of the mixing frequency ⁇ 1- ⁇ 2 is achieved by a special narrow band of the laser lines for the optical discrete frequencies ⁇ 1, ⁇ 2, so that the mixing frequency ⁇ 1- ⁇ 2 is defined very precisely.
  • the stability of the mixing frequency ⁇ 1- ⁇ 2 is achieved by stabilizing the DFB laser diodes. In the present case, either each of the optical frequencies ⁇ 1, ⁇ 2 can be stabilized or the distance ⁇ 1- ⁇ 2 of the optical frequencies ⁇ 1, ⁇ 2 can be stabilized by a corresponding control circuit.
  • the optical discrete frequency ⁇ 1, ⁇ 2 of the same can be adjusted by regulating the temperature and / or the current of the DFB laser diodes, in order directly to the mixing frequency ⁇ 1 - ⁇ 2 to a stable value.
  • All mixed frequencies ⁇ 1- ⁇ 2, ⁇ 1- ⁇ 3, ⁇ 2- ⁇ 3 of the CW-THz radiation 40 formed from the discrete optical frequencies ⁇ 1, ⁇ 2, ⁇ 3 are guided in the spectrometer 100, 200 simultaneously through a sample P in the CW-THz radiation guidance ie
  • the further mixing frequencies ⁇ 1- ⁇ 3, ⁇ 2- ⁇ 3 of the CW-THz radiation 40 are received. Measurement data of the sample P at the other mixing frequencies ⁇ 1- ⁇ 3, ⁇ 2- ⁇ 3 are thus determined simultaneously with the reference measurement.
  • a photocurrent of the stable mixing frequency ⁇ 1- ⁇ 2 is continuously determined with a photocurrent of the further tunable mixing frequencies ⁇ 1- ⁇ 3 and ⁇ 2- ⁇ 3 - i.
  • a reference measurement is available for the measured data.
  • the average value of the phase cp 0 and amplitude A 0 of a CW-THz radiation determined at the beginning T 0 of a measurement is substantially stable at the frequency-stabilized mixing frequency ⁇ 1- ⁇ 2 and at best due to Drift changes of the spectrometer developed; these can due to z.
  • phase cp1 and amplitude A1 of a CW-THz radiation determined at a later time T1 of a measurement on the frequency-stabilized mixing frequency ⁇ 1- ⁇ 2 substantially substantially reflects dynamic influences on the mechanical components of the spectrometer and expressly no sample-specific development phase ⁇ and amplitude A of a CW-THz radiation 40 (under constant conditions for the sample).
  • phase ⁇ and amplitude A of a CW-THz radiation 40 on the frequency-stabilized mixing frequency ⁇ 1- ⁇ 2 also for the phase ⁇ and amplitude A of a CW-THz radiation 40 in the further mixed frequencies ⁇ 1- ⁇ 3 and ⁇ 2- ⁇ 3 is present.
  • the phases ⁇ and amplitudes A of the CW-THz radiation 40 determined as a result of the measurement data of the further mixing frequencies ⁇ 1- ⁇ 3 and ⁇ 2- ⁇ 3 are insofar as the drift-related development of the phase ⁇ and amplitude A of a CW-THz radiation 40 at the frequency-stabilized mixing frequency correct ⁇ 1- ⁇ 2.
  • the CW-THz spectrometer 100, 200 have an evaluation unit 50 with a module for determining a drift of the phase and amplitude ⁇ 0 , A 0 , ⁇ , , A, of the THz radiation 40 at the stabilized mixing frequency ⁇ 1- ⁇ 2.
  • the module is designed to determine a temporal drift profile of the phase and amplitude ⁇ ,, A, over the entire measuring time and then to determine an average value of the phase and amplitude cp 0 , A 0 from the drift profile.
  • a tunable single third optical discrete frequency ⁇ 3 is provided. This can be used to form the further tunable CW THz frequencies ⁇ 1- ⁇ 3 and ⁇ 2- ⁇ 3. With the two different tunable mixing frequencies ⁇ 1- ⁇ 3 and ⁇ 2- ⁇ 3 to form a respective CW-THz frequency, the measuring time of a single spectrum can be halved.
  • the third optical frequency ⁇ 3 can first be selected approximately midway between the frequencies ⁇ 1 and ⁇ 2 and then tuned in a suitable range.
  • the CW-THz spectrometer 100, 200-as well as the CW-THz spectrometer 300, 400 of FIG. 2- has one with the lasers L1, L2, L3 or L1, L2 L4 via a control line S or S '. connected to the adjustment unit 60, which is designed to receive a THz spectrum, the at least one further mixing frequency ⁇ 1- ⁇ 3 or ⁇ 1- ⁇ 3, ⁇ 2- ⁇ 3 or ⁇ 2- ⁇ 3, in the range of the THz spectrum to detune and thereby keep the stabilized mixing frequency ⁇ 1- ⁇ 2 constant.
  • the spectral resolution of a CW-THz spectrometer 100, 200, 300, 400 basically depends initially on the linewidths of the radiation characteristics of the lasers L1, L2 and L4.
  • the CW-THz spectrometer 200 which varies from the embodiment of FIG. 1, it is provided to replace the laser L3 by a laser L4 designed as a multi-mode laser, which generates a multiplicity of discrete optical frequencies.
  • an optical path length difference OPD generated by the optical path delay unit 23 in the range of about 1 cm may be sufficient to spectrally separate the signal information in the photocurrent associated with the individual frequencies ⁇ 3.
  • the embodiment of a CW-THz spectrometer 200 as shown in FIG. 1 with an alternative laser L3 to the laser L3, can be used to obtain an amplitude independent of this amplitude at a moderate amplitude of the path length difference (eg 1 cm) To obtain spectral resolution, which depends only on the characteristics of the laser.
  • FIG. 2 shows CW-THz spectrometers 300, 400 which, like the CW-THz spectrometers 100, 200 of FIG. 1, are designed according to the principle similar to a Mach-Zehnder interferometer for a dispersive Fourier transform spectroscopy are.
  • the variant of a CW THz spectrometer 300 is also possible with an easily tunable laser L3.
  • the optical part of the structure of the CW-THz spectrometers 300, 400 is completely fiber-based and due to the complete fiber guide 20 comparatively compact.
  • the setup allows a particularly short measurement time with a large bandwidth and good spectral resolution - so holds a high application potential.
  • the structure is robust and stable, since in particular the free jet of the CW-THz radiation 40 is not affected.
  • a Fourier transformation in the context of the evaluation unit 50 (not shown) can be replaced by trigonometric interpolation or regression. Thus, a higher resolution is possible with the same delay line.
  • the CW-THz spectrometer 300 provides for the use of three single-mode lasers, L1, L2, L3 and the CW-THz spectrometer 400, respectively, provides for the use of two single-mode lasers, L1, L2 and a multi-mode laser.
  • Mode laser L4 before, in both cases, a reference measurement or a correction measure of an amplitude and phase of a CW-THz radiation with high precision can always be recorded simultaneously with a measurement spectrum.
  • the CW-THz spectrometers 300, 400 use in comparison to the CW-THz spectrometers 100, 200 have a modulator arrangement 25 with a first fiber stretcher 25.1 in the first branch 21 of the CW-THz spectrometer 300, 400 and with a second fiber stretcher 25.2 in the second branch 22 of the CW-THz spectrometer 300, 400. These replace the modulator in the form of the optical path delay unit 23 in the CW-THz spectrometers 100, 200.
  • the phase of the mixing frequencies ⁇ 1 - ⁇ 2 or ⁇ 1 - ⁇ 3 or ⁇ 2- ⁇ 3 generated by mixing the optical discrete frequencies ⁇ 1, ⁇ 2, ⁇ 3 of the optical radiation is varied by a piezo-driven fiber stretcher 25.1, 25.2 of the modulator arrangement 25.
  • the length of an optical fiber of the fiber guide 20 is about 60m, which is wound for the most part on a cylindrical piezoelectric body.
  • a change in the circumferential length of the piezoelectric body produces a change in the optical path length OPD of the fiber.
  • such a change in the optical path length in the manner described above is also possible with the optical delay path of the path delay unit 23, but not with the presently high scanning rate.
  • a variation and / or modulation of the optical path length OPD of 3 mm with a modulation rate of approximately 1 kHz is possible.
  • These can also be adapted to the respective needs. It has been shown that the acquisition of a CW-THz spectrum up to 2 THz with a resolution of 1 GHz requires only about 10 minutes of measurement time. If, on the other hand, a relatively coarse resolution of 20 GHz is sufficient, 30 seconds are enough.
  • the asymmetric operation of the fiber stretcher 25.1, 25.2 additionally increases the path length difference generated in the first branch 21 and in the second branch 22.
  • the sensitivity to thermal variations is reduced because they have the same effect in both paths - thermal variations or other slow dynamic drifts in the fiber stretchers are almost completely compensated.
  • the fiber stretcher can be operated with a bipolar voltage up to ⁇ 400 V at 300 Hz. Higher modulation rates are possible at lower amplitudes.
  • the first branch 21 and the second branch 22 can be built exactly symmetrically, so that the fiber stretcher 25.1, 25.2 can be operated with inverted voltage.
  • FIG. 4 shows in view (A) and view (B) a measurement result in which the fiber stretcher 25.1, 25.2 are modulated with a frequency of about 800 Hz at a bipolar voltage amplitude of 85 V, ie produce an optical path length difference OPD of 3 mm , At a CW THz frequency of 1 THz, this allows a phase shift of 10 * 2 * ⁇ , ie an effective modulation frequency of about 8 kHz.
  • Figure 4 (A) shows the voltage of the fiber stretters 25.1, 25.2 and the THz photocurrent at the receiver 32 as a function of time for a THz frequency of 600 GHz.
  • the rate of voltage modulation was chosen for 800 Hz and the photocurrent was averaged over 240 periods corresponding to a period of 300 ms.
  • the data were taken at an open aperture of 10 mm diameter, ie without sample.
  • the sinusoidal voltage V of the fiber stretcher and the detected THz photocurrent is shown in both cases as a function of time.
  • the length that is to say the photocurrent
  • the photocurrent is modulated only comparatively slowly, which is clearly shown in the image in FIG. 4 (A).
  • Fig. 4 (B) the expected exact cosine behavior of the photocurrent can be seen when it is plotted against the stress of the fiber stretcher.
  • the combination of a multi-mode laser L4 with discrete equidistant voice frequencies-preferably with a line spacing of about 25 GHz and the two fiber stretchers 25.1, 25.2 - makes it possible to detect a broadband CW-THz spectrum at a rate of a few hundred Hz up to 1, 2 kHz. Depending on the desired spectral resolution, this rate is limited by the set stroke of the modulator arrangement to achieve the desired amplitude of the optical path length delay OPD and the desired resolution.
  • the two fiber stretcher 25.1, 25.2 can be arranged asymmetrically to shorten the measurement time and increase the spectral resolution.
  • FIG. 5 shows a particularly preferred embodiment of a THz spectrometer 500 constructed similarly to a Mach-Zehnder interferometer according to a first variant of the invention in which coherent THz radiation 40 is generated directly at a THz frequency ⁇ 7 ⁇ 2 .
  • the THz radiation 40 is shown symbolically in a radiation guide 520 with a radiation course which is dashed as a line.
  • the THz source 531 here has a source I of the first type for generating the THz frequency Cl ⁇ and a second-type source II for generating the THz frequency ⁇ 2 .
  • the THz source 531 - here several BWO oscillators - has the function of a transmitter 31, which as well as other identical or similar features or features identical or similar function with the same reference numerals as in FIG. 1 and Fig. 2 is provided.
  • the THz radiation at the THz frequencies ⁇ 2 is fed in a symbolically represented radiation guide 520 to a beam splitter 521 which transmits the THz radiation 40 at the THz frequencies ⁇ 2 ⁇ 2 to a first branch 21 and a second branch 22 of the THz radiation.
  • Spectrometer 500 feeds.
  • the THz radiation guide 520 with the first and second branch 21, 22 is in the present case designed as free-jet guidance with beam splitters 521, 522 and mirrors 41, 42.
  • a sample P may be arranged, which is transilluminated by the THz radiation 40 at THz frequencies ⁇ , ⁇ 2 .
  • the THz radiation 40 of the first branch 21 21 after the sample P carries information about the dielectric function of the sample P and is guided in the direction of passage of another beam splitter 522 onto a receiver 32.
  • the THz radiation 40 of the first branch is superimposed homodyne with the THz radiation 40 of the THz frequencies ⁇ 1, ⁇ 2 of the second branch 22.
  • the second Branch 22 is a modulator 23 provided as a path delay unit PD, by means of which a phase difference between the THz radiation 40 from the second branch 22 and the THz radiation 40 from the first branch 21 is variable. In this way, a receiver signal can be detected in the receiver 32 equipped with a detector as a function of the varied phase difference.
  • the detector is presently formed as an intensity detector, z. B. in the form of a bolometer by means of an interference-dependent intensity at the receiver 32 to form the receiver signal is detectable.
  • a phase and amplitude of the THz radiation 40 is detected in order to obtain the complete information about the dielectric function of the sample P.
  • the detector is connected to an evaluation unit 50, which is designed to determine the phase and amplitude of the THz radiation 40 at the frequency-stabilized THz frequency C 1 as the reference frequency and the at least one further THz frequency ⁇ 2 from the receiver signal and together to compare. Similar to the ones described with reference to FIG. 1 and FIG.
  • a frequency stabilizing means is provided, which is able to keep the frequency-stabilized THz frequency Cl ⁇ constant as a reference frequency at least over a measurement period.
  • the THz source 531 have an active or passive frequency stabilization to keep the frequency-stabilized THz frequency Cl ⁇ constant over the entire measurement period as a reference frequency.
  • the at least one further THz frequency ⁇ 2 for receiving a THz spectrum for the sample P can be varied, for example, in a range between 0.05 and 1.8 THz.
  • the width of the THz measurable spectrum depends on the THz frequency ⁇ 2 variation available through the THz source 531.
  • Both the well-tuned THz frequency ⁇ 2 and the correspondingly-frequency-stabilized THz frequency C 1 are thus simultaneously conducted in the THz radiation guide 520 of the CW-THz spectrometer 500.
  • a simultaneous reference measurement at the frequency-stabilized THz frequency Cl 1 is thus available.
  • the reference measurement is used as a correction measure become .
  • a phase or amplitude measured at the further THz frequency ⁇ 2 is corrected by means of a time-dependent value of the phase ⁇ , or amplitude A, at the frequency-stabilized THz frequency C 1 or an average value ⁇ 0 , A 0 thereof.
  • the THz spectroscopy method with the THz spectrometer 500 in particular makes it possible to determine the drift correction at the same time via the reference measurement.
  • a constant, in particular frequency-stabilized THz frequency is performed in the spectrometer 500, which is available as a reference frequency for determining a correction measure for amplitude and phase.

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Abstract

L'invention concerne un spectromètre THz (100) comprenant: - un ensemble source (10) pour générer un rayonnement optique d'au moins une première et une deuxième fréquence optique (ω1, ω2); - un guide de rayonnement optique qui est couplé à l'ensemble source (10) et qui présente une première et une deuxième branche (21, 22) pour le guidage du rayonnement optique de toutes les fréquences, ladite première branche (21) présentant un mélangeur opto-électronique sous la forme d'un émetteur (31) apte à générer un rayonnement THz (40) à partir des fréquences de mixage des fréquences optiques de la première branche (21) et ladite deuxième branche (22) présentant un mélangeur opto-électronique se présentant sous la forme d'un récepteur (32) et apte à recevoir le rayonnement THz (40); - un ensemble modulateur (25) comprenant au moins un modulateur (23) qui permet de modifier une différence de phase entre un signal d'une fréquence de mixage des fréquences optiques de la deuxième branche (22) et le rayonnement THz (40); - un détecteur qui permet de détecter un signal de récepteur en fonction de la différence de phase modifiée pour déterminer au moins une phase, en particulier une phase et/ou une amplitude du rayonnement THz (40), en particulier à l'aide d'un détecteur de photocourant qui permet de détecter un photocourant dans le récepteur (32) pour la formation du signal de récepteur.
PCT/EP2011/064758 2010-09-07 2011-08-26 Spectromètre thz et procédé de spectrométrie thz WO2012031917A1 (fr)

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Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP2998714A1 (fr) * 2014-09-19 2016-03-23 Philipps-Universität Marburg Spectromètre THz
CN114414522A (zh) * 2021-12-27 2022-04-29 中国科学院上海微系统与信息技术研究所 采用太赫兹光学自探测表征光频梳相干光谱的装置和方法

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CN114414522A (zh) * 2021-12-27 2022-04-29 中国科学院上海微系统与信息技术研究所 采用太赫兹光学自探测表征光频梳相干光谱的装置和方法
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